The only pigment synthesized in most animal species, and the only pigment at all in mammalian species, is melanin. There are two classes of melanin: pheomelanin, which produces a blond or red color, and eumelanin, which produces a dark brown or black color. Both classes of melanin are synthesized from tyrosine, but their synthetic pathways diverge after production of dopaquinone.
The primary switch controlling whether a particular melanocyte produces pheomelanin or eumelanin is the melanocortin receptor (MC1R). MC1R polymorphisms also appear to be the primary determinant of red or blond pheomelanin.
The melanocortin receptor can be activated by any of the melanocyte-stimulating hormones (MSH), most commonly by α-MSH, but also by β-defensin 103. Conversely, activation of MC1R can be inhibited through expression of the agouti signaling protein (ASP). The β-defensin 103 signal is dominant over the ASP signal and the ASP signal is dominant over the MSH signal. Further, there are mutations and polymorphisms in all of these genes that increase or decrease their activity. Coat color is further modulated by multiple modulators of melanin production and transport, including tyrosinase (TYR), the tyrosinase transporter OCA2, and the ubiquitination gene HERC2, among others.
Plants make multiple additional classes of pigments, including chlorophyll, carotenoids, anthrocyanins, and betalains. Of these pigments, carotenoids can be most easily transferred to animals because production of carotenoid dyes in plants relies on a precursor that can also be found in animals. Geranylgeranyl pyrophosphate is an intermediate in the HMG-CoA reductase pathway, and is used as a precursor for synthesizing steroids and sterols. With the addition of 4-5 plant proteins, geranylgeranyl pyrophosphate can instead be used as a precursor for synthesizing carotene yellow and orange, or torulene red. The transfer of plant pigments to animals has occurred in nature: the aphid A. pisum has several genes somehow transferred from fungi. Carotenoids have also been produced in genetically-engineered yeast, which belong to the animal kingdom.
The only modification of surface pigment ever attempted by genetic intervention in multicellular animals is the wholesale change of animals from light to dark or dark to light. No more detailed patterns have been created. Carotene dyes have never been naturally found in animals more complicated than aphids, and have never been engineered into any multicellular animal. Before the present invention, customizable color or patterns in the skin or fur of animal species have not been created.